The proposed research is a continuation of ongoing work on the membrane systems (sarcoplasmic reticulum [SR] and transverse-axial[T-axial] tubules of various mammalian species. Primarily through the use of electron miocroscopy (TEM, SEM and HVEM), the investigations are designed to describe the process of formation of membrane systems and characterize the components of both forming and mature membrane systems. Structures of particular interest are the couplings, complexes formed by apposition of SR with the sarcolemma (T-axial tubules or surface sarcolemma). The SR that contributes to couplings (junctional SR, J-SR) generates structures of varying morphology that appear to connect the J-SR and apposed sarcolemma. These structures (known variously as "junctional processes", "SR feet", or "pillars") may be adhesive; alternatively, or as well, they may serve as electrical contacts whereby action potentials are relayed to the J-SR from the sarcolemma or T-axial tubules. There is currently much interest in junctional processes of skeletal and smooth muscle, and it is important that the structural parallels which exist in cardiac muscle be documented. Particular aspects being examined are the forms taken by myocardial junctional processes and patterns of their arrangement on the J-SR. Other projects that concern SR ultrastructure include investigations of development and distribution of specialized segments of the so-called "network" SR (N-SR) which surrounds each myofibril. It appears, for example, that the morphology and distributional density of N-SR varies at different levels of the myocardial cell. A number of projects that concern the T-axial tubular system are being carried out. These include the description of large tubulovesicular proliferations ("labyrinths") derived from T-axial tubules and found to date in mouse and shrew hearts. The contributions in terms of volume and surface area that labyrinths make to the cells in which they appear will be determined, and their formation traced in developing mouse heart. The mechanism of formation of T-axial tubules will be compared in the mouse and guinea pig, two mammals in which the final form of the T-axial tubular system varies considerably. Finally, the images of fractured or torn myocardial cells prepared for SEM will be evaluated by correlative TEM in order to determine whether T-axial tubules and SR are truly being visualized.